AP Bio Study Guide & Review Unit 3 ReviewCellular Energetics

AP Biology Unit 3, Cellular Energetics, is about how cells capture, store, and release energy to stay alive, and it makes up 12-16% of the AP exam. The single biggest idea is energy flow: living systems constantly take in energy, convert it through metabolic pathways like photosynthesis and cellular respiration, and lose some as heat to keep their order intact. Enzymes run every reaction in those pathways, so the unit ties molecular structure straight to the chemistry that powers life.

What this unit covers

Enzymes and how the environment changes them

• Enzymes are proteins that act as biological catalysts. They speed up reactions by lowering the activation energy, the energy hump a reaction needs to clear to get going.
• The substrate fits the enzyme's active site by shape and charge, forming an enzyme-substrate complex. If the fit is wrong, the reaction doesn't happen.
• Denaturation happens when temperature, pH, or the chemical environment disrupts the protein's structure (often by breaking hydrogen bonds), which warps the active site and shuts down catalysis.
• Every enzyme has an optimal temperature and pH. Outside that range, activity drops because the shape gets distorted.
• Raising temperature speeds molecules up, so enzymes and substrates collide more often and the reaction rate climbs, until you hit the optimal point and denaturation takes over.
• The relative amounts of substrate and product control how efficiently the reaction runs. Competitive inhibitors bind reversibly to the active site and block the substrate from getting in.

Energy, thermodynamics, and ATP

• All living systems need a constant energy input. Life is highly ordered, but it doesn't break the laws of thermodynamics because energy input has to exceed energy loss to keep that order going.
• Cells couple reactions. An exergonic reaction that releases energy (like glucose breakdown) powers an endergonic reaction that requires energy (like building proteins).
• A big enough loss of order or energy flow means death. There's no maintaining a cell without that steady input.
• ATP (adenosine triphosphate) is the cell's energy currency. Splitting off a phosphate to form ADP plus inorganic phosphate releases energy the cell can spend, and the cell recharges ADP back to ATP over and over.

Photosynthesis: capturing light energy

• Photosynthesis uses CO2, H2O, and light energy to make carbohydrates and O2. Photosynthetic organisms turn sunlight into sugars they can use or store.
• It first evolved in prokaryotes. Cyanobacterial photosynthesis is what oxygenated Earth's early atmosphere, a huge deal for the evolution of aerobic life.
• In the chloroplast, chlorophyll in photosystems I and II absorbs light and boosts electrons to higher energy levels. Water splits to replace the lost electrons, releasing O2.
• Electrons move down an electron transport chain in the thylakoid membrane and are eventually handed to NADP+, reducing it to NADPH in photosystem I.

Cellular respiration: releasing stored energy

• Cellular respiration uses energy stored in macromolecules to make ATP. Respiration and fermentation occur in all forms of life.
• Glycolysis happens in the cytosol. It breaks glucose down to make ATP (from ADP and inorganic phosphate), NADH (from NAD+), and pyruvate.
• Pyruvate moves into the mitochondrion and is oxidized. The Krebs (citric acid) cycle runs in the mitochondrial matrix, releasing electrons that reduce NAD+ to NADH and FAD to FADH2, and releasing CO2.
• The electron transport chain runs a series of oxidation-reduction reactions that build an electrochemical gradient across the membrane. That gradient drives ATP synthesis.

Shared pathways and common ancestry

• Core metabolic pathways like glycolysis and oxidative phosphorylation are conserved across all three domains: Archaea, Bacteria, and Eukarya.
• That shared chemistry is evidence for common ancestry. Every living thing runs basically the same energy software.

Unit 3, Cellular Energetics at a glance

Why Unit 3, Cellular Energetics matters in AP Bio

This unit is where the chemistry from earlier units turns into something a living cell actually does. It connects structure to function at the molecular level and explains how every cell stays ordered against the pull of entropy. Energy flow is one of the course's biggest recurring threads, and this is where you learn the actual machinery.

• It shows energetics in action: how cells obey thermodynamics while still building and maintaining complex order.
• Enzymes here are your case study for how protein structure determines function, a theme that comes back constantly.
• The conservation of glycolysis and oxidative phosphorylation across all domains is direct evidence for evolution and common ancestry.
• Tracing carbon and electrons through these pathways builds the modeling and reasoning skills the exam rewards everywhere.

How this unit connects across the course

• Builds directly on the macromolecules and water chemistry from Unit 1 (Chemistry of Life). Enzymes are proteins, ATP is a nucleotide derivative, and hydrogen bonds are what denaturation breaks.
• Depends on the organelles and membranes from Unit 2 (Cells). Chloroplasts, mitochondria, and the membranes that hold electron transport chains are why these pathways have a place to happen.
• Feeds into Unit 4 (Cell Communication and Cell Cycle), where energy and signaling regulate cellular activity, and many signal pathways rely on ATP and feedback like the regulation you see in enzymes.
• Sets up Unit 8 (Ecology), where photosynthesis and respiration drive energy flow through ecosystems and the global carbon cycle, scaling this molecular story up to whole food webs.

Key equations and processes

• Photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. Use it whenever you trace where carbon and oxygen go when cells store light energy.
• Cellular respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. The reverse logic of photosynthesis; it releases the stored energy.
• ATP hydrolysis: ATP → ADP + Pi + energy. Spend this energy to power endergonic work like synthesis, active transport, and movement.
• Glycolysis: glucose → 2 pyruvate, producing ATP and NADH in the cytosol, no oxygen required.
• Krebs (citric acid) cycle: pyruvate-derived carbon is oxidized in the matrix, producing NADH, FADH2, and CO2.
• Electron transport chain and chemiosmosis: NADH and FADH2 drop electrons through redox reactions to build an electrochemical gradient that drives ATP synthesis.

Essential questions

• How do cells stay highly ordered without breaking the laws of thermodynamics?
• How does the structure of an enzyme determine whether a reaction happens, and how does the environment shut it down?
• How is energy captured from light, stored in chemical bonds, and later released to power cellular work?
• Why does the same core metabolism showing up in every domain of life count as evidence for common ancestry?

Key terms to know

• Activation energy: The energy barrier a reaction must clear to proceed; enzymes lower it.
• Enzyme-substrate complex: The temporary structure formed when a substrate binds an enzyme's active site by matching shape and charge.
• Denaturation: Loss of an enzyme's functional shape when temperature, pH, or chemicals disrupt its structure.
• Competitive inhibitor: A molecule that reversibly binds the active site and blocks the substrate.
• Exergonic reaction: A reaction that releases energy and runs spontaneously.
• Endergonic reaction: A reaction that requires an energy input and is not spontaneous.
• ATP: Adenosine triphosphate, the cell's main energy currency, recharged from ADP and inorganic phosphate.
• Electron transport chain: A series of oxidation-reduction reactions in a membrane that builds an electrochemical gradient.
• Electrochemical gradient: The stored difference in charge and concentration across a membrane that drives ATP synthesis.
• Glycolysis: The cytosol pathway that splits glucose into pyruvate, making ATP and NADH.
• Krebs cycle: The matrix reactions that oxidize carbon, producing NADH, FADH2, and CO2.
• Photosystem: A cluster of chlorophyll and proteins that absorbs light and boosts electrons to higher energy.
• NADPH: The reduced electron carrier made in the light reactions of photosynthesis.
• Autotroph: An organism that builds organic compounds from inorganic ones using light or chemical energy.

Common mix-ups

• Photosynthesis and respiration are not exact opposites running in the same cells in the same way. Plants do both; respiration releases the energy that photosynthesis stored, but they happen in different organelles with different carriers.
• NADH and NADPH are different molecules. NADH carries electrons in respiration; NADPH carries them in photosynthesis. Don't swap them.
• Higher temperature does not always speed up an enzyme. It increases rate up to the optimal point, then denaturation drops activity off a cliff.
• Enzymes lower activation energy. They do not change whether a reaction is exergonic or endergonic, and they aren't used up in the reaction.